The goal of this project is to understand the molecular mechanisms by which gene expression is regulated in the unicellular eukaryote, the yeast Saccharomyces cerevisia. Cell specialization in yeast is determined by the mating type locus, which is subject to mating type interconversion catalyzed by the product of the HO gene. This gene is under exquisite control: expressed only in haploid cells. only in mother cells, and only in the G1 phase of the cell cycle. This proposal uses biochemical and genetic techniques to seek a molecular understanding of how HO is regulated. Prior work has identified eleven genes whose products control expression of HO. The SWI genes (SWI1-SW16) code for activator proteins--necessary for transcription of HO. The SIN genes (SINI-SIN5) code for negative regulators, perhaps transcriptional repressors, some of which are proposed to be antagonized by the Swi proteins. A particular focus of this proposal is cell-cycle regulation of HO: a sequence responsible for cell-cycle regulation (the "cell cycle box") and a protein complex (the "cell-cycle box factor") that binds to this sequence have been identified. This project shall determine the way in which this complex participates in cell cycle regulation and the way in which it is regulated by a chain of interactions involving a negative regulator (Sinl), which is inhibited by an activity termed Swi1,2,3, which is itself inhibited by the Sin3 activity. The overall goal of this proposal is to understand how these Swi and Sin proteins work and how they and other proteins are responsible for regulating HO and other genes. There are two broad specific aims: (1) to determine the mechanism by which Swi1, Swi2, and Swi3 proteins activate transcription of HO, whether this activity is regulated by Sin3, and whether it regulates other yeast genes; (2) to determine the mechanism of cell cycle regulation by the cell cycle box factor, the way in which the Cdc28 protein kinase governs this process, and how this activity is regulated by Sin1. This project is health related in that it provides information on regulation of the cell division cycle, which is crucial to understanding the mechanism of growth control.